Effects of interfacial adhesion on the rubber toughening of poly(vinyl chloride) Part 2. Low-speed tensile tests

Abstract

The influence of interfacial adhesion on the tensile properties, including toughness, of poly(vinyl chloride) (PVC)–nitrile rubber (NBR) blends with the morphology of well-dispersed rubber particles has been investigated using two types of blends. The first type, which contains NBR26 (NBR with 26 wt% acrylonitrile (AN)), has a higher interfacial adhesion strength than the second type that contains NBR18. The secant modulus and yield stress of the blends were found to be independent of interfacial adhesion. On the other hand, the elongation-at-break and toughness (defined as the area under the stress–strain curve to break) depend strongly on the interfacial strength. The effects of rubber particle size, size distribution and rubber volume fraction on the tensile properties have been combined into the effect of a single morphological parameter, the matrix ligament thickness T. Both the elongation-at-break and toughness increase as T decreases. At $\text{T<0.06}\text{μm}$ the blends with stronger interfacial adhesion (PVC–NBR26) have much higher elongation-at-break and toughness. Stress whitening was observed in all deformed PVC–NBR18 blends. For PVC–NBR26 blends, however, stress whitening occurred only at $\text{T>0.06}\text{μm}\text{.}$ Transmission electron microscopy studies revealed that debonding at the PVC–NBR interface is the sole microvoiding mechanism that causes stress whitening.

Introduction

The toughness of a plastic can be enhanced substantially by blending with rubber particles [1]. Both morphological parameters and interfacial adhesion between the matrix and the dispersed particles play important roles in controlling the toughness of the blends. It is commonly believed that the effects of morphological parameters and interfacial adhesion are interrelated. For instance, strengthening the interfacial adhesion leads to a better dispersion of particles and smaller particle size and hence a change of toughness. This effect of interfacial adhesion on toughening is termed indirect [2]. On the other hand, interfacial adhesion may have a direct effect on toughening when the morphological parameters are identical [2].

At constant interfacial adhesion, the toughness of plastic–rubber blends measured at low speeds of deformation is a function of morphological parameters, i.e. rubber particle size, volume fraction and particle spatial distribution parameter. Muratoglu and coworkers [3] demonstrated that there is a substantial influence of both rubber particle size and volume fraction on the elongation-at-break and toughness of nylon 6-rubber blends. The toughness (defined as the area under the stress–strain curve to break) was found to increase with increasing rubber content for a constant rubber particle size and with decreasing rubber particle size for a constant rubber content. Takaki and coworkers [4] reported that the effects of both the size and volume fraction of methyl methacrylate–butadiene–styrene graft copolymer (MBS) particles on the elongation-at-break and the toughness of poly(vinyl chloride) (PVC)–MBS blends can be combined into the effect of a single parameter, namely the matrix ligament thickness. These properties were found to increase with decreasing matrix ligament thickness. Kim and coworkers [5] reported that the rubber particle spatial distribution has a substantial influence on the elongation-at-break values of styrene–acrylonitrile copolymer (SAN)–rubber blends.

However, interfacial adhesion has been reported to have little direct effect on the toughness of the polymer–rubber blends at low strain rates. Dompas and coworkers [6] investigated the effect of interfacial adhesion on the stress–strain curves of the PVC–MBS blends by using a constant volume fraction of MBS particles of size of 0.25 μm. They concluded that the interfacial adhesion has no influence on the stress–strain behavior of the blends. Huang and coworkers [7] found that interfacial adhesion has only a small effect on the fracture behavior of rubber-toughened epoxies. Cho and coworkers [8] investigated the effect of interfacial adhesion on the fracture toughness of the poly(methyl methacrylate) (PMMA)–rubber blends determined by three point-bending tests. By studying the fracture toughness as a function of the rubber particle size at a constant rubber content, they concluded that interfacial adhesion does not have any influence.

The interfacial adhesion between PVC and nitrile rubber (NBR) can be strengthened through an increase of the acrylonitrile (AN) level in NBR in the range of 0–40% by weight [9]. Imasawa and Matsuo [10] studied the strain rate dependence of the elongation-at-break for PVC–NBR blends with various AN contents. The NBR phase as well as the PVC phase in these blends were continuous [9]. However, it was not possible to separate the effects of morphological parameters and interfacial adhesion for blends with co-continuous phases due to lack of a theoretical framework.

In the first paper of this series, we reported the direct influence of interfacial adhesion on the impact strength of PVC–NBR blends with the morphology of well-dispersed rubber particles [2]. In the present work, we use the same PVC–NBR blends with two different levels of interfacial adhesion to investigate the direct effect of interfacial adhesion on the toughness of the blends determined by the low-speed tensile measurements. Microvoiding mechanisms, such as crazing [1], internal cavitation of rubber particles [6], [11] and debonding between the matrix and dispersed particles [2], [12], [13], [14], relieve the triaxial dilatational stresses ahead a crack tip, thereby promoting plastic deformation. Therefore, microvoiding is one of the most important toughening mechanisms. In this paper, the role of microvoiding in the toughening of the PVC–NBR blends is also discussed.